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Among the most difficult problems for those interested in understanding the structural biology of amyloidoses are how to determine the complete three-dimensional structure of amyloid fibrils and to monitor the protein folding events leading to formation of these end-stage assemblies. X-ray diffraction approaches have yielded only modest amounts of information because of the inability to crystallize amyloid fibrils. However, great strides recently have been made in the application of NMR techniques to this problem. In two papers published online in Nature Structural Biology on 22 April 2002, the Goto and Radford groups report exciting new results that promise to significantly extend our ability to elucidate the structural dynamics of amyloid formation.

Both labs studied the amyloid formed by β2-microglobulin (β2m), a 99-residue protein associated with hemodialysis-related amyloidosis. The Goto group used H/D exchange to map protected residues in the amyloid fibril. The Radford group used urea denaturation to determine individual residue stabilities, establishing four distinct behavioral groups based on denaturation profiles. The results from both studies were remarkably consistent. A large portion of the β-sheet comprising the native β2m monomer was involved in forming the core of the amyloid fibril. Less stable regions in the native fold, including the N- and C-termini and two edge β-strands, had lower protection factors and lower stability in urea. These areas may thus be important in initiating the amyloidogenic transformation of the native protein. The experimental approaches used in these studies should be applicable to other amyloidogenic proteins, allowing new insights about the structural dynamics of their assembly.

What about Aβ? Kheterpal et al., 2000 have shown that H/D exchange measurements with mass spectroscopy can provide information about the organization of the Aβ fibril core. Therefore, much could be learned about Aβ fibril structure using the new techniques. Whether an equivalent amount can be learned about individual stages of Aβ folding and assembly is unclear. Aβ is a peptide, not a protein. It has no disulfide bonds nor is it known to have a stable, native structure in vivo, as do β2m, transthyretin, and many other amyloidogenic proteins. The conformational transitions occurring during Aβ amyloid assembly thus are unlikely to involve the same degree of reorganization of existing, stable, secondary structure elements as seen in proteins. Nevertheless, it will be interesting and important to do the experiments to see just how much we can learn. Based on the Goto and Radford reports, there is every reason to be optimistic.